DISCUSSION OF THE PRIOR ART
FIG. 10 shows the configuration of an existing optical scanner and a light beam printer using this scanner. The scanner uses polygonal mirror 31, which is in the shape of a regular polygon whose outer edges are coated with mirror surfaces 31a, 31a, etc. Polygonal mirror 31 is rotated at a fixed angular velocity by DC servomotor 32, which is controlled by driver circuit 35. Laser beam .alpha., emitted by semiconductor laser device 36, is focused by imaging lens 33 and projected onto one of the mirror surfaces 31a of polygonal mirror 31. When laser beam .alpha. is reflected off mirror surface 31a, it passes through beam scanning lens 34 and strikes the surface of, for example, a light sensitive drum 37. For a code reader, the drum 37 is replaced by a medium to be read and a focusing lens, not shown, is provided between the scanning lens 34 and read medium.
When polygonal mirror 31 is rotating at a fixed angular velocity, the angle at which laser beam .alpha. strikes mirror surface 31a will vary, and consequently the direction in which laser beam .alpha. is reflected will also vary. In this way the laser beam .alpha. can scan the surface of, for example, light-sensitive drum 37.
With this type of optical scanner, a polygonal mirror and a DC servomotor to drive its rotation are required. This has made it very difficult to reduce the size of the optical scanner, and it has placed a limit on how much size reduction is possible. Furthermore, in order to achieve precision in the width of the area scanned, the scanning speed and other scanning characteristics a high degree of accuracy is required in the dimensions of the mirror surfaces on the polygonal mirror, the angle at which each pair of surfaces meets, and so on. Accordingly, processing costs and the cost of assembly and adjustment are quite high and difficult to reduce. Another problem is that the angular range of scanning is fixed for each optical scanner, as the angle over which the laser beam scans is determined by the number of mirror surfaces. It is thus impossible to alter the angular range of scanning.
Furthermore, existing optical scanners are capable of scanning a laser beam in one direction only. A single scanner is incapable of switching between different directions of scanning or of scanning in two directions simultaneously.